The present invention relates to semiconductor structures, and particularly to semiconductor structures including semiconductor devices having a programmable floating backplate, methods of manufacturing the same, and methods of operating the same.
In semiconductor-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) circuits, a conductive region can be provided underneath a buried oxide (BOX) layer. Such a conductive region is typically referred to as a backplate. A conductive electrical contact is provided to the backplate, typically by a contact via that extends from a top surface of an SOI substrate to the backplate. An appropriate voltage potential can be applied to the backplate through the contact via to alter the characteristics of SOI devices and circuits above the BOX layer. The backplate is externally biased, i.e., is not a floating node.
For example, in the case of fully-depleted SOI CMOS circuits, it is desirable to tune the threshold voltages of n-type field effect transistor (nFET) devices and p-type field effect transistor (pFET) devices to achieve substantially the same magnitude. Such adjustments can provide a balance between the nFET and pFET drive currents, which leads to improved device performance and circuit speed. A negative voltage applied to the backplate can be used to simultaneously increase the threshold voltage of the nFET and reduce the threshold voltage of the pFET. Alternatively, a positive voltage applied to the backplate can be used to simultaneously increase the threshold voltage of the pFET and reduce the threshold voltage of the nFET.
In the case of CMOS devices, the device characteristics can be influenced by electrical charges trapped in the BOX if the density of the trapped electrical charge is sufficiently high. Trapped electrical charge in the BOX layer can be the result of a CMOS chip fabrication process. For example, irradiative processes or high-ion-energy processes, such as reactive-ion etching or plasma deposition, can result in electrical charges trapped in the BOX layer. The effect of undesirable electrical charges trapped in the BOX layer can be “neutralized” by applying a bias voltage to a backplate located on the backside of the BOX layer. Thus, a positive backplate voltage can be used to neutralize the effect of negative charge in the BOX and a negative backplate voltage can be used to neutralize the effect of positive charge in the BOX.
Backplates employed in known structures for SOI devices and circuits are hard-wired backplates, i.e., non-floating backplates that are not electrically isolated from other structures. Such hard-wired backplates are electrically biased through an electrically conductive path that includes a contact structure through a BOX layer. Each backplate has an electrical wire connection, and the plate voltage is applied via the wire connection.